In recent years, aluminum has been replaced by copper as the material mainly employed in metal interconnecting films for semiconductor devices. In the case of aluminum films, titanium nitride films are formed as barrier films at the interface between the aluminum films and silicon substrates. However, these titanium nitride films are poor in it's ability to prevent the diffusion of copper. Thus, W.sub.x N films (tungsten nitride films) have attracted attention as barrier films against copper films.
It has been a practice to produce W.sub.x N films at a high temperature (i.e., 500.degree. C. or above) under high pressure (i.e., film-forming pressure: several thousand Pa). However, a large-scale apparatus is employed to sustain such a high pressure; and, in addition, troublesome operations are needed for the maintenance thereof. In a pretreatment apparatus for forming W.sub.x N films and a film forming apparatus for forming copper film on the W.sub.x N films, substrates should be treated in vacuum. Thus, there arises an additional problem that these apparatuses are poor in connection properties with a W.sub.x N film forming apparatus; and thus, the substrates cannot be treated continuously.
Accordingly, it has been required to develop a film forming apparatus by which W.sub.x N films can be produced in vacuum (under reduced pressure). In FIG. 5(a), a substrate 120, on which a W.sub.x N film and a copper film are to be formed, consists of a silicon substrate 150, a silicon oxide film 152 formed on the silicon substrate 150 and a pore 160 formed in the silicon oxide film 152.
When a W.sub.x N film is to be formed on the substrate 120 by using a CVD apparatus 102 of the prior art as shown in FIG. 6, a reactor 111 is first evacuated. Then, the substrate 120 is carried thereinto and placed on a holder 114 provided in the bottom side of the reactor 111.
A shower nozzle 112 is provided in the ceiling side of the reactor 111. After heating the substrate 120 to a prescribed temperature with a heater contained in the holder 114, two types of feedstock gases (for example, WF.sub.6 gas and NH.sub.3 gas) are jetted from the shower nozzle 112 toward the substrate 120 as shown by arrows 151, thereby inducing the following chemical reaction: EQU 4WF.sub.6 +8NH.sub.3.fwdarw.2W.sub.2 N+24HF+3N.sub.2.
Thus, a W.sub.x N film 153 is formed on the surface of the substrate 120 as shown in FIG. 5(b), wherein X is referred tentatively as to 2.
When a prescribed thickness of the W.sub.x N film is achieved, the substrate 120 is taken out from the reactor 111. Then, a copper film 154 is formed on the W.sub.x N film 153 as shown in FIG. 5(c) and followed by the transportation to the subsequent stage, i.e., the patterning of the copper film 154, etc.
When the W.sub.x N film 153 and the copper film 154 are formed in a vacuum as described above, the substrate 120 can be continuously treated without exposing to the atmosphere by connecting an apparatus for forming a tungsten film and a apparatus for forming a copper film to a multi-chamber type apparatus.
However, a CVD apparatus of the prior art as described above suffers from the problem of serious dusting. This is because the reaction between WF.sub.6 and NH.sub.3 proceeds even at room temperature, and not W.sub.x N but WF.sub.6.4NH.sub.3 etc. are formed at room temperature, different from the above reaction formula, and adhere to the inner wall of the reactor 111.
When the wall of the reactor 111 is heated to a temperature close to the temperature of the substrate 120, at least the formation of WF.sub.6.4NH.sub.3 can be prevented. In this case, however, W.sub.x N is deposited on the inner wall of the reactor 111 on the contrary and causes dusting.
In addition, the above-described reactor 111 of the prior art suffers from another problem of a low growth speed of the W.sub.x N film. Thus, it has been required to clarify the cause of this phenomenon and to establish an effective countermeasure.